A major difficulty in extracting oil from deposits of heavy, viscous oils or from tar sand deposits results from the poor mobility of the oil and the requisite movement through the deposit and into an oil well. A number of different techniques and apparatus have been developed for reducing the viscosity of the oil, usually by increasing its temperature. In many instances this is accomplished by electrical heating, including particularly conductive heating of a portion of the oil producing formation or "pay zone" adjacent to the well.
One such method employs a primary heating electrode in ohmic contact with the pay zone. When a voltage differential is established between that electrode and the pay zone, electrical current flows; the current density may be quite high in the immediate vicinity of the primary electrode. As a consequence, a part of the oil producing formation immediately around the wellbore is heated; this reduces tee viscosity and subsequently reduces the excessive pressure drop around the well bore. By so doing, the flow rate of the well can be increased and the ultimate recovery from the reservoir is increased, since less pressure is wasted.
For economical operation of a well heating system of this type, electrical power may be delivered to the primary heating electrode through the conventional metal oil well casing, usually a steel pipe. If efficient heating is to be realized, this requires electrical insulation of the casing from the earth. But most electrical insulating materials, when buried in moist earth, ca only function reasonably well for short periods during which the added capacitance created by the penetration or absorption of moisture into the insulation does not significantly affect performance of the system.
In a power delivery system for heavy-oil well heating, the moisture absorbing capability of casing insulation can seriously degrade performance by radically increasing the capacitance and often the leakage current, between the well casing and the earth. This increases the shunt capacitive reactance currents along the casing and can result in considerable inefficiency. Insulating materials are available which resist moisture absorption (e.g. polyethylene) but many such moisture resistant materials lack the physical or chemical properties needed for oil well processes.